CODEWITHSUNDEEP
c++multithreadingqt
Kęstutis
Kęstutis

Reputation: 1061

How to connect a signal from a thread to a slot?

What I simply want to do is connect a signal inside a thread to a slot in the main thread to handle UI changes.

This is basically the current state of my thread, nothing fancy but it's just for testing purposes atm:

// synchronizer.h
class Synchronizer : public QObject
{
    Q_OBJECT

public:
    Synchronizer();

signals:
    void newConnection(std::wstring id);

private:
    QTimer timer;

private slots:
    void synchronize();
}

// synchronizer.cpp
Synchronizer::Synchronizer()
{
    connect(&timer, SIGNAL(timeout()), this, SLOT(synchronize()));
    timer.start();
}

void Synchronizer::synchronize()
{
    emit newConnection(L"test");
}

And here's how my MainWindow looks:

// mainwindow.h
namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
    Synchronizer synchronizer;

private slots:
    void addConnection(std::wstring id);
}

// mainwindow.cpp
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(&synchronizer, SIGNAL(newConnection(std::wstring)),
            this, SLOT(addConnection(std::wstring)));
    QThread *thread = new QThread;
    // The problems starts here?
    synchronizer.moveToThread(thread);
    thread->start();
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::addConnection(std::wstring id)
{
    // Add a new connection to QListWidget
    ui->connectionList(QString::fromStdWString(id));
}

If I remove there lines:

synchronizer.moveToThread(thread);
thread->start();

everything seems to work as expected, that is a new item is added every second to a QListWidget but as soon as I move the synchronizer object to thread it simply stops working. I'd presume it has something to do with the connect context but I'm not really sure how something like this should be achieved as I'm quite new to Qt.

Upvotes: 0

Views: 470

Answers (1)

Kęstutis
Kęstutis

Reputation: 1061

It seems that the in this case was simply the fact that I am using std::wstring as an argument in the signal without registering the type first and after adding the following line qRegisterMetaType<std::wstring>("std::wstring"); to the code, everything worked as expected.

If I would have read the output console more carefully I would have solved the problem without too much hassle as it was clearly stated that:
QObject::connect: Cannot queue arguments of type 'std::wstring'

So simply speaking, read the compiler output and don't be stupid like me :)

Upvotes: 1

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